3',4'-Dihydroxyflavonol (DiOHF) is a cardioprotective flavonol that can reduce injury after myocardial ischemia and reperfusion and thus is a promising small molecule for the treatment of cardiovascular disease. Like all vasoactive flavonols reported to date, DiOHF is both relaxant and antioxidant, hindering investigation of the relative contribution of each activity for the prevention of reperfusion injury. This study investigates structure-activity relationships of variations at the 3' and 4' positions of the B ring of DiOHF and vasorelaxant and antioxidant activities. Relaxation of rat isolated aortic rings precontracted with KCl revealed that the most active flavonols were those with a 4'-hydroxyl group, with the opening of potassium channels as a possible contributing mechanism. For the antioxidant activity, with the exception of DiOHF, none of the flavonols investigated were able to significantly scavenge superoxide radical, and none of the three most potent vasorelaxant flavonols could prevent oxidant-induced endothelial dysfunction. The discovery of single-acting vasorelaxant flavonols without antioxidant activity, in particular 4'-hydroxy-3'-methoxyflavonol, will assist in investigating the mechanism of flavonol-induced cardioprotection.
Glucuronosyl diacylglycerides (GlcAGroAc2) are functionally important glycolipids and membrane anchors for cell wall lipoglycans in the Corynebacteria. Here we describe the complete synthesis of distinct acyl-isoforms of GlcAGroAc2 bearing both acylation patterns of (R)-tuberculostearic acid (C19:0) and palmitic acid (C16:0) and their mass spectral characterization. Collision-induced fragmentation mass spectrometry identified characteristic fragment ions that were used to develop "rules" allowing the assignment of the acylation pattern as C19:0 (sn-1), C16:0 (sn-2) in the natural product from Mycobacterium smegmatis, and the structural assignment of related C18:1 (sn-1), C16:0 (sn-2) GlcAGroAc2 glycolipids from M. smegmatis and Corynebacterium glutamicum. A synthetic hydrophobic octyl glucuronoside was used to characterize the GDP-mannose-dependent mannosyltransferase MgtA from C. glutamicum that extends GlcAGroAc2. This enzyme is an Mg(2+)/Mn(2+)-dependent metalloenzyme that undergoes dramatic activation upon reduction with dithiothreitol.
Two-component systems (TCSs) are key elements in bacterial signal transduction in response to environmental stresses. TCSs generally consist of sensor histidine kinases (SKs) and their cognate response regulators (RRs). Many SKs exhibit autokinase, phosphoryltransferase and phosphatase activities, which regulate RR activity through a phosphorylation and dephosphorylation cycle. However, how SKs perform different enzymatic activities is poorly understood. Here, several crystal structures of the minimal catalytic region of WalK, an essential SK from Lactobacillus plantarum that shares 60% sequence identity with its homologue VicK from Streptococcus mutans, are presented. WalK adopts an asymmetrical closed structure in the presence of ATP or ADP, in which one of the CA domains is positioned close to the DHp domain, thus leading both the -and -phosphates of ATP/ADP to form hydrogen bonds to the "-but not the -nitrogen of the phosphorylatable histidine in the DHp domain. In addition, the DHp domain in the ATP/ADP-bound state has a 25.7 asymmetrical helical bending coordinated with the repositioning of the CA domain; these processes are mutually exclusive and alternate in response to helicity changes that are possibly regulated by upstream signals. In the absence of ATP or ADP, however, WalK adopts a completely symmetric open structure with its DHp domain centred between two outward-reaching CA domains. In summary, these structures of WalK reveal the intrinsic dynamic properties of an SK structure as a molecular basis for multifunctionality.
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